Ignoring non-existent master UFOs from designspace v5?

Would it be possible to ignore UFOs that are source fonts in a design space (v5) but don’t exist (yet)?

https://github.com/googlefonts/fontmake/blob/d9185063e152586c180336fd5fad4158144b4748/Lib/fontmake/font_project.py#L129-L133

I am building an upright and an italic VF from a designspace file. At the time the upright font is built, the italic source UFOs don't exist yet*. They are not needed to build the upright font, but it is currently not possible to build the font because fontmake insists on loading all source UFOs before the actual build starts.

*My build process is orchestrated by make, so some tasks may run in parallel, and the italic UFOs appear at a random time in the directory.